Conventionally, various starters for cranking an internal combustion engine have been known. Typically, in such a starter, as disclosed in Japanese patent laid open publication No. 58-53857, U.S. Pat. Nos. 4,604,907, 4,561,316, 4,573,364, 4,520,285, 4,510,406, and 4,528,470, a planetary gear reduction unit is provided between a DC motor and a pinion to transmit the driving force of the DC motor to the pinion for driving a ring gear of the engine.
In an engine starter system having such a structure, it is necessary to prevent the rotation of the internal gear of the planetary reduction gear unit relative to its center bracket, and to mount the center bracket on the casing so as not to rotate relative to the casing. According to the conventional structure, the internal gear is prevented from rotating relative to the casing by providing engagement projection and recess structures between the internal gear and the center bracket as well as between the center bracket and the casing. However, the need for the two engagement projection and recess structures complicates not only the structure of the center bracket but also the overall structure.